In recent years, there has been a demand for freely controlling an optical frequency of an optical signal in order to, for example, increase a transmission capacity and improve flexibility of a network. However, in devices such as an optical vector modulator and an acousto-optic switch, a shift band of the optical frequency is limited and it is difficult to shift the optical frequency of the optical signal to another wavelength grid. Furthermore, a polarized multiplexed optical signal is difficult to handle in such devices. In view of this, opto-electro-optic conversion is performed to generate an optical signal of a desired optical frequency.
For example, in an optical add-drop multiplexer, when there is an optical signal desired to be changed only in optical frequency, the optical signal desired to be changed only in the optical frequency is outputted to a drop port to be converted into an electric signal, then the electric signal is again converted into an optical signal with the optical frequency changed, and the optical signal thus obtained is inputted from an add port. In short, in the optical add-drop multiplexer, it is possible to shift the optical frequency of the optical signal by performing the opto-electro-optic conversion.
Meanwhile, in a wavelength division multiplex (WDM) device, WDM signals are subjected to opto-electronic conversion and combined in an optical transmitter of a new optical frequency grid. In other words, in the WDM device, it is possible to shift the optical frequencies of the WDM signals and combine the WDM signals by performing the opto-electro-optic conversion.
The conventional techniques described above are described in Japanese Laid-open Patent Publication Nos. 4-45428 and 2004-93583.